The invention relates to a circuit arrangement for detecting the vertical blanking periods in a picture signal, comprising a pulse interval detector with means for receiving a composite synchronizing signal contained in the picture signal and comprising pulses in a predetermined sequence, and means for supplying a change in its output signal when the time intervals between consecutive pulses fall short of a predetermined time threshold, and a pulse shaping stage with means for receiving the output signal and which, in response thereto produces a vertical synchronizing signal.
A picture signal, for example a television signal, as a rule contains a plurality of types of pulse-shaped synchronizing signals which are superposed on each other and on the signals which contain the picture content to form the picture signal. The total of all the superposed synchronizing signals is denoted as the composite synchronizing signal, which consists of line synchronizing pulses, alternatively denoted as the horizontal pulses, and field pulses, alternatively denoted as the vertical pulses. A horizontal pulse is inserted at the transition between two lines of the picture signal, while at the transition between two fields, usually a plurality of vertical pulses occur, the exact number of which depends on the transmission standard of the picture signal. Immediately prior to and after the vertical pulses there is a predetermined number of equalizing pulses which are designated as pre-equalizing pulses and post-equalizing pulses and are significantly narrower than the horizontal and vertical pulses.
The U.S. Pat. No. 4,258,389 discloses a circuit for recovering a vertical synchronizing signal from a composite synchronizing signal, which comprises a pulse interval test circuit with monostable multivibrators and a D-type flip-flop, and also a signal shaping circuit with monostable multivibrators. The pulse interval test circuit is preceded by a pulse width test circuit comprising monostable multivibrators and a D-type flip-flop. The pulse width test circuit samples all the pulses of the composite synchronizing signal applied thereto and produces a signal when the width of the sampled pulse falls short of three quarters of the target value of the width of the horizontal pulses. This signal is applied to the pulse interval test circuit which examines the composite synchronizing signal to check whether, at the end of a half-line period, a further pulse occurs. If so, the D-type flip-flop of the pulse interval test circuit changes state, as a result of which the monostable multivibrators of the signal shaping circuit are again driven so as to supply pulses which constitute the vertical synchronizing signal recovered from the circuit.
In the prior art circuit arrangement, the pulse width and spacings to be checked are compared with pulse-shaped signals which ae derived, without exception, from monostable multivibrators. Such multivibrators are, as regards their function, basically analog circuits which are temperature-dependent to a large extent. In the prior art circuit arrangement of a plurality of time intervals determined by monostable multivibrators because of the stringing, errors in the multivibrators may accumulate in an impermissible manner and cause faulty operation. Consequently, the circuit arrangement must be adjusted very carefully. In addition, because the capacitors are always required for the monostable multivibrators to determine their time constants, the circuit arrangement cannot, or only at high cost and design efforts, be integrated on a semiconductor wafer. Furthermore the prior art circuit arrangement produces a vertical synchronizing signal which does not coincide in time with the vertical pulses of the composite synchronizing signal.